Various tire pressure monitoring systems are known which operate either on the basis of directly measuring sensors or detect an abnormal tire pressure by evaluating rotational speed properties or oscillation properties of the vehicle wheels.
DE 100 58 140 A1, which is incorporated by reference, discloses what is referred to as an indirectly measuring tire pressure monitoring system which detects a tire pressure loss by evaluating the rotational movement of the wheel (DDS: Deflation Detection System).
EP 0 578 826 B1, which is incorporated by reference, discloses a tire pressure gauge which determines a pressure loss in a tire on the basis of tire oscillations, wherein at least one resonant frequency component is extracted from the tire oscillations.
A method for indirectly monitoring tire pressure which, by taking into account the natural torsional frequency of the tires, improves an indirectly measuring tire pressure monitoring system which is based on the evaluation of the rotational movement of the wheel is disclosed in patent application DE 10 2005 004 910 A1, which is incorporated by reference.
A tire monitoring system with combined evaluation of wheel speed information and analysis of the axial frequency in which the vertical acceleration of the wheels is measured and taken into account by means of sensors is described in WO 03/031990 A1, which is incorporated by reference.
Indirect systems for detecting loss of tire pressure which are known from the prior art resort to the wheel speed information in order to calculate an indicator variable for a tire pressure loss (pressure loss detection variable), wherein the change in the natural frequency or resonant frequency of the torsional oscillation between the rim and the belt in the event of a pressure loss is used as an indicator variable which can be observed in a frequency spectrum of the wheel speed signal. This change is based on the reduction in the torsional spring constant between the belt and the rim when there is reduced pressure in the tire. The displacement/change in the resonant frequency of a current frequency spectrum of the wheel speed compared to a resonant frequency learnt in the case of correct tire air pressure is then used to detect the pressure loss.